Method for auto-ignition operation and computer readable storage device

ABSTRACT

The invention relates to an internal combustion engine comprising a fuel injector ( 2 ) for each cylinder; a fuel injection control unit ( 4 ) for controlling fuel injection quantity and a piston ( 5 ) in each cylinder whose compression action causes a mixture of air and fuel to be ignited. The engine is further provided with inlet and outlet valves ( 6, 7 ) and various sensors ( 12 - 16 ) for measuring various engine operating parameters. During compression ignition mode, the control unit ( 4 ) is arranged to select a λ-value from a map stored in the control unit, which value is a function of engine load and engine speed, and to compare the actual λ-value with the selected λ-value; whereby the control unit is arranged to adjust the intake manifold pressure as a function of the difference between the said λ-values in order to obtain the selected λ-value. The invention further relates to a method for operating the engine and a computer readable storage device ( 4 ).

[0001] The present application claims priority to European PatentApplication No. 02029091.2, filed Dec. 30, 2002, titled INTERNALCOMBUSTION ENGINE, METHOD FOR AUTO-IGNITION OPERATION AND COMPUTERREADABLE STORAGE DEVICE, naming Hans Ström and Lucien Koopmans asinventors, and claims priority to European Patent Application No.02029060.7, filed Dec. 30, 2002, titled INTERNAL COMBUSTION ENGINE,METHOD FOR AUTO-IGNITION OPERATION AND COMPUTER READABLE STORAGE DEVICE,naming Hans Ström and Lucien Koopmans as inventors, the entire contentsof which are incorporated herein by reference.

BACKGROUND AND TECHNICAL FIELD

[0002] The invention relates to an internal combustion engine that canbe operated in a homogeneous charge compression ignition combustionmode, as well as a method for controlling such an engine.

DETAILED DESCRIPTION

[0003] To improve thermal efficiency of gasoline internal combustionengines, lean burn is known to give enhanced thermal efficiency byreducing pumping losses and increasing ratio of specific heats.Generally speaking, lean burn is known to give low fuel consumption andlow NO_(x) emissions. There is however a limit at which an engine can beoperated with a lean air/fuel mixture because of misfire and combustioninstability as a result of a slow burn. Known methods to extend the leanlimit include improving ignitability of the mixture by enhancing thefuel preparation, for example using atomised fuel or vaporised fuel, andincreasing the flame speed by introducing charge motion and turbulencein the air/fuel mixture. Finally, combustion by auto-ignition, orhomogeneous charge compression ignition, has been proposed for operatingan engine with very lean or diluted air/fuel mixtures.

[0004] When certain conditions are met within a homogeneous charge oflean air/fuel mixture during low load operation, homogeneous chargecompression ignition can occur wherein bulk combustion takes placeinitiated simultaneously from many ignition sites within the charge,resulting in very stable power output, very clean combustion and highfuel conversion efficiency. NO_(x) emission produced in controlledhomogeneous charge compression ignition combustion is extremely low incomparison with spark ignition combustion based on propagating flamefront and heterogeneous charge compression ignition combustion based onan attached diffusion flame. In the latter two cases represented byspark ignition engine and diesel engine, respectively, the burnt gastemperature is highly heterogeneous within the charge with very highlocal temperature values creating high NO_(x) emission. By contrast, incontrolled homogeneous charge compression ignition combustion where thecombustion is uniformly distributed throughout the charge from manyignition sites, the burnt gas temperature is substantially homogeneouswith much lower local temperature values resulting in very low NO_(x)emission.

[0005] Engines operating under controlled homogeneous charge compressionignition combustion have already been successfully demonstrated intwo-stroke gasoline engines using a conventional compression ratio. Itis believed that the high proportion of burnt gases remaining from theprevious cycle, i.e., the residual content, within the two-stroke enginecombustion chamber is responsible for providing the hot chargetemperature and active fuel radicals necessary to promote homogeneouscharge compression ignition in a very lean air/fuel mixture. Infour-stroke engines, because the residual content is low, homogeneouscharge compression ignition is more difficult to achieve, but can beinduced by heating the intake air to a high temperature or bysignificantly increasing the compression ratio. This effect can also beachieved by retaining a part of the hot exhaust gas, or residuals, bycontrolling the timing of the intake and exhaust valves.

[0006] In all the above cases, the range of engine speeds and loads inwhich controlled homogeneous charge compression ignition combustion canbe achieved is relatively narrow. The fuel used also has a significanteffect on the operating range; for example, diesel and methanol fuelshave wider auto-ignition ranges than fuel. A further problem is toachieve ignition at a particular time with maintained combustionstability, while avoiding engine knocking, misfiring and increasedNO_(x) levels.

[0007] When the engine is operating in HCCI mode, a low fuel consumptionhas to be sustained and or optimised. For low fuel consumption, theair/fuel ratio has to be greater than stoichiometric, i.e. substantially1 part fuel and 14 parts air. There is a trend for lower fuelconsumption in the direction of higher values of lambda if goodcombustion stability can be sustained. A three way catalyst onlyconverts unburned hydrocarbons and carbon monoxide when the engine isoperating lean (λ>1), hence NO_(x) emissions will be emitted to theatmosphere without after-treatment.

[0008] High lambda values for optimised fuel consumption and NO_(x)emissions can not be achieved with atmospheric pressure in the inletmanifold in combination with the inlet and exhaust valve settings usedfor HCCI combustion.

[0009] Hence an object of the invention is to provide a means forcontrolling the combustion process during auto-ignition, in order tomaintain low NO_(x) levels. Said means allows for monitoring of currentcombustions and for correction of subsequent combustions dependent onthe outcome of the monitoring process.

[0010] The above problems can be solved, in some cases, by anarrangement, a method and a computer readable storage device forcontrolling homogeneous charge compression ignition combustion, asdescribed in more detail below.

[0011] One embodiment relates to an internal combustion enginepreferably, but not necessarily, provided with variable valve timing(VVT), cam profile switching (CPS), direct fuel injection (DI), andmeans for boosting the manifold absolute pressure (turbo, compressoretc.).

[0012] The following text will be mainly concentrated on embodimentsincluding the above features. However, the general principle of theinvention as claimed is also applicable to, for instance, stationaryaspirating engines with fixed valve timing and a standard camshaft. Suchengines are often operated at fixed speeds and loads and are not subjectto the transients normally occurring in, for instance, engines forvehicles.

[0013] Also, although the following examples relate to fuels, an engineoperating according to principles of the invention can be adapted to usemost commonly available fuels, such as kerosene, natural gas, andothers.

[0014] The engine is possible to be operated on homogeneous chargecompression ignition (HCCI) combustion mode. This is a combustion mode,different than conventional spark ignited (SI) combustion mode, in orderto reduce fuel consumption in combination with ultra low NO_(x)emissions. In this mode, a mixture containing fuel, air and combustionresiduals is compressed with a compression ratio between 10.5 and 12 toauto ignition. The HCCI combustion has no moving flame front, incontradiction to a SI combustion that has a moving flame front. The lackof a flame front reduces temperature and increases the heat release ratehence increases the thermal efficiency of the combustion. The combustionresiduals are captured when operating the engine with a negative valveoverlap. Residuals increase the temperature of the mixture so that theauto ignition temperature is reached before piston top dead center (TDC)and dilute the mixture so that the heat release rate decreases to anacceptable level. By controlling the heat release, cycle-to-cyclevariations (COV), noise and knocking combustion can be reduced. Thenegative valve overlap is achieved when the exhaust valve is closedbefore piston TDC and the inlet valve is opened after piston TDC in thegas exchange phase of the engine cycle.

[0015] The acquired valve timing for the negative overlap can beachieved by using VVT and CPS, hence switching from conventional SIvalve timing to HCCI valve timing with a shorter the valve openingduration and/or valve lift

[0016] In HCCI combustion mode one target is to keep NO_(x) emissionsbelow the legislative limit by using a control loop that controls theHCCI combustion. NO_(x) emissions, fuel consumption and combustionstability are affected by the air fuel ratio of the mixture in thecombustion chamber. A low air fuel ratio gives high fuel consumption andhigh NO_(x) emissions, while a high air fuel ratio gives low fuelconsumption and low HC emissions.

[0017] According to one embodiment of the invention, an internalcombustion engine is provided with at least one cylinder and arranged tobe switched between spark ignition mode and compression ignition mode.The engine comprises a fuel injector, through which fuel is injectedinto a combustion chamber, for each cylinder and a fuel injectioncontrol unit that controls fuel injection quantity per combustion cycleinjected through each fuel injector. Fuel injection is preferably, butnot necessarily, achieved by means of direct injection (DI) into eachcombustion chamber. For the current invention, port injection is alsopossible.

[0018] A spark may be sustained in HCCI mode in order to keep the sparkplug from fouling and, although the gas mixture is arranged to selfignite, contribute to an increased combustion stability and avoidance ofmisfire.

[0019] A reciprocating piston is arranged in each engine cylinder whosecompression action causes a mixture of air and fuel within thecombustion chamber to be ignited. Gas exchange is controlled by at leastone inlet valve preferably, but not necessarily, provided with variablevalve timing per cylinder for admitting a combustible gas, such as air,and at least one exhaust valve preferably, but not necessarily, providedwith variable valve timing per cylinder for exhausting combusted gases.

[0020] The combustion process is monitored by sensors for measuring airintake manifold pressure and the air/fuel ratio, or λ-value, for thecombusted exhaust gas, as well as standard sensors for engine load andengine speed.

[0021] Other sensors may include temperature sensors for engine coolantand oil as well as NO_(x)-sensors and ion current sensors. If required,the engine may also be provided with engine knocking and combustionstability sensors. The knock sensor can be of the piezo-electric type,which may also be used for continuous monitoring of cylinder pressure.The combustion stability sensor may be an acceleration type sensor, suchas a flywheel sensor, or an ion current sensor. Alternatively, both saidsensors can be replaced by a single in-cylinder piezoelectric pressuresensor. By processing the output from such a sensor, it is possible toobtain a signal representing engine knock and a signal representingengine stability.

[0022] According to a further embodiment the engine is switched into acompression ignition mode. The control unit is arranged to select aλ-value from a map of λ-values stored in the control unit, which valueis a function of engine load and engine speed. The actual λ-value iscompared to the selected λ-value; whereby the control unit is arrangedto adjust the intake manifold pressure as a function of the differencebetween the said λ-values in order to obtain the selected λ-value.

[0023] High λ-values combined with low fuel consumption and low NO_(x)emissions can not be achieved with atmospheric pressure in the intakemanifold during HCCI operation at high loads and/or speeds. In oneembodiment the engine is provided with an intake air charging unit,arranged to adjust the intake manifold pressure.

[0024] The intake air charging unit may be a turbocharger provided witha wastegate for controlling the intake manifold pressure. Alternativelythe intake air charging unit is a supercharger provided with a throttlefor controlling the intake manifold pressure.

[0025] According to a further embodiment, the intake air charging unitis arranged to increase the intake manifold pressure if the actualλ-value is less than the selected λ-value. Similarly, the intake aircharging unit is arranged to decrease the intake manifold pressure ifthe measured actual λ-value is greater than or equal to the selectedλ-value.

[0026] According to a further embodiment, the intake manifold isprovided with a sensor for measuring intake air temperature. As theλ-value may vary with intake air temperature, the control unit isarranged to adjust the selected λ-value if necessary. The adjustment isbased on the measured intake air temperature compared to a referencetemperature for the stored map.

[0027] According to a further embodiment, the invention relates to acomputer readable storage device having stored therein data representinginstructions executable by a computer to implement a compressionignition for An internal combustion engine, the engine having a pistondisposed in a cylinder to define a combustion chamber, intake valves foradmitting fresh air into the cylinder, a fuel injector for injectingfuel into the combustion chamber, exhaust valves for discharging exhaustgas resulting from combustion within the cylinder, wherein opening andclosing timings of the intake means and opening and closing timings ofthe exhaust means are adjustable, and sensors for measuring engine load,engine speed and an actual λ-value for the exhaust gas.

[0028] The computer readable storage device comprises:

[0029] instructions for adjusting opening and closing timings of theintake means and opening and closing timings of the exhaust means suchthat the piston reciprocates within the cylinder to perform an exhaustphase, an exhaust gas retaining phase, an intake phase, a compressionphase, and an expansion phase;

[0030] instructions for providing a first start time of a first fuelinjection by the fuel injector during said exhaust gas retaining phaseand a second start time of a second fuel injection by the fuel injectorduring said exhaust gas retaining phase;

[0031] instructions for selecting a λ-value from a map stored in thecontrol unit, which value is a function of engine load and engine speed;

[0032] instructions for determining a difference between the actualλ-value with the selected λ-value; and

[0033] instructions for adjusting the intake manifold pressure as afunction of the difference between the said λ-values in order to obtainthe selected λ-value

[0034] The computer readable storage device further comprisesinstructions for determining intake manifold pressure control signalsindicative of the adjustment required by an intake air charging unit, asdetermined by the control unit on the basis of comparison between themeasured actual λ-value with the selected λ-value.

[0035] The storage device further comprises instructions for determiningintake manifold temperature by means of a temperature sensor and foradjusting the λ-values in said stored map with respect to the measuredtemperature.

[0036] According to one embodiment, the engine is arranged to switchfrom SI-mode to HCCI-mode when certain operating parameters arefulfilled. During compression ignition mode, the exhaust valve isarranged to close before top dead center during an exhaust stroke of thepiston and the intake valve is opened after top dead center during asuction stroke of the piston. This creates a period of negative valveoverlap, during which exhaust and intake valves are closed. The fuelinjection control unit is arranged to control the fuel injectionquantity so as to perform a first fuel injection before top dead centerof the piston stroke and to perform a second fuel injection after topdead center of the piston stroke during the interval when both of theexhaust and intake valves are closed. However, other examples of this socalled split injection timing are also possible.

BRIEF DESCRIPTION OF DRAWINGS

[0037] In the following text, further embodiments will be described indetail with reference to the attached drawings. These drawings are usedfor illustration only and do not in any way limit the scope of theinvention. In the drawings:

[0038]FIG. 1 shows a schematic internal combustion engine;

[0039]FIG. 2 shows a diagram illustrating the variation of cylinderpressure over crank angle for HCCI- and SI-mode;

[0040]FIG. 3 shows a diagram illustrating the variation of lambda inrelation to engine load and engine speed in HCCI-mode;

[0041]FIG. 4 shows a diagram for a lambda control loop for combustioncontrol in HCCI-mode.

[0042] FIGS. 5A-B shows diagrams of an engine provided with differenttypes of intake air charging devices.

[0043]FIG. 1 shows a schematic illustration of an internal combustionengine according to the invention. The engine is provided with at leastone cylinder 1 and comprises a fuel injector 2, through which fuel isinjected into a combustion chamber 3, for each cylinder. A fuelinjection control unit 4 controls fuel injection quantity per combustioncycle injected through each fuel injector. A piston 5 in the enginecylinder has a compression action that causes a mixture of air and fuelwithin the combustion chamber to be ignited during HCCI-mode. Thecylinder is provided with at least one inlet valve 6 for admitting gaswhich includes fresh air into said cylinder and at least one exhaustvalve 7 for exhausting combusted gases from said cylinder. Air issupplied through an intake conduit 9 connected to an intake manifold,while exhaust gas is exhausted through an exhaust conduit 10. DuringSI-mode, the ignition of the fuel/air mixture is ignited by a spark plug8.

[0044] The control unit receives signals from at least one sensor formeasuring engine operation parameters, which sensors may include acombustion chamber pressure sensor 11, an intake manifold pressuresensor 12 and a λ-probe 13 in the exhaust conduit, as well as atemperature sensor for intake air 14, an engine load sensor 15 and anengine speed sensor 16. The control unit controls the intake and exhaustvalves 6, 7 by means of valve actuators 17, 18. The actuators may beeither electrically or mechanically operated.

[0045] According to an alternative embodiment, the λ-probe 13 can bereplaced or supplemented by a NO_(x)-sensor or an ion current sensor,which generates a signal indicative of the λ-value. The function of thelatter sensors will be described in connection with FIG. 4 below.Similarly, the sensors for engine load and speed 15, 16 may be replacedor supplemented by temperature sensors for engine coolant and engine oilFIG. 2 shows a diagram illustrating the variation of cylinder pressureover crank angle for HCCI- and SI-mode. As can be seen from the curvesin the diagram, the engine can be operated in homogeneous chargecompression ignition (HCCI) combustion mode and in conventional sparkignited (SI) combustion mode. The HCCI combustion has no moving flamefront, as opposed to a SI combustion that has a moving flame front. Thelack of a flame front reduces temperature and increases the heat releaserate hence increases the thermal efficiency of the combustion. This willresult in a considerably higher peak pressure after ignition (IG);typically in excess of 40 bar, as opposed to about 20 bar in SI mode.The main difference between the HCCI- and SI modes is that a part of thecombustion residuals are captured by operating the engine with anegative valve overlap. The negative valve overlap is achieved byclosing the exhaust valve, or EV, before piston TDC (EVC) and openingthe inlet valve, or IV, after piston TDC (IVO) in the gas exchange (GE)phase of the combustion, as illustrated in FIG. 2. During the air intakephase, residuals increase the temperature of the mixture so that theauto ignition temperature is reached before piston top dead center (TDC)and dilutes the mixture so that the heat release rate decreases to anacceptable level. By controlling the heat release, noise and knockingcombustion can be reduced.

[0046] A split fuel injection is used having a pilot direct fuelinjection (PI) before TDC during the negative valve overlap and a maindirect fuel injection (MI) after TDC of the negative valve overlap. Therelative quantities of fuel injected during the pilot and the maininjections can be varied and are calculated and controlled by a fuelinjection control unit (not shown). The fuel of the pilot injection (PI)will react with the retained residuals, forming intermediates orcombustion products. This reaction can be exothermic hence heating theresiduals, resulting in earlier timing of the auto ignition temperature.The total quantity of injected fuel for the pilot and the main injectionis substantially constant with respect to the current engine speedrequirements. The quantity of the first injection is selected to be inthe range of 10-45% of the total amount of injected fuel.

[0047] The above example describes a split fuel injection occurringbefore and after top dead center of the piston stroke during theinterval when both of the exhaust and intake valves are closed. However,the invention is not limited to this particular embodiment of splitinjection timing.

[0048] Due to the demand for dilution, which controls the rate of heatrelease, only the part load regime of the engine is used for HCCIcombustion mode. The auto ignition timing for HCCI operation can becontrolled by the pilot fuel injection and/or the captured amount ofresiduals and/or the absolute manifold pressure. The latter iscontrolled by increasing or decreasing the pressure of the intake air bymeans of a compressor or turbocharger.

[0049] According to one embodiment, the amount of trapped residualsduring negative overlap should be in the range 20-60%, irrespective ofhow this is achieved.

[0050] When operating the engine, it is desirable to avoid engineknocking, low combustion stability and a high noise level. Knocking,which is also a source of noise, is detected by measuring the peakpressure and/or pressure variations caused by a too rapid heat releaseduring the expansion phase. Knocking occurs when the peak pressureexceeds an expected maximum pressure, or when a series of rapid pressurevariations occur during the expansion phase. Low combustion stability isindicated by high cycle to cycle variations of the pressure duringcombustion. Typically, an engine operated in HCCI mode may oscillatebetween a late phased combustion (low cylinder pressure) and asubsequent early phased combustion (high cylinder pressure). However,the above conditions are of less importance for the current invention.

[0051]FIG. 3 illustrates a schematic λ map to be stored in a controlunit. As seen from the figure, the air/fuel ratio λ is approximately=1.5at idling speed under a low load. If either the load or the engine speedis increased, while the other parameter is kept substantially constant,then the air/fuel ratio is increased to approximately λ=2. When bothengine speed and engine load are increased according to the linearfunction shown, then the air/fuel ratio is increased to approximatelyλ=2.3.

[0052]FIG. 4 shows a schematic diagram for a control strategy formanaging the NO_(x) emissions by adjusting the absolute pressure in theair intake manifold. The adjustments are made based on a comparisonbetween the measured, actual lambda value and a lambda value selected onthe basis of a number of sensor readings. It is not possible to adjustthe intake manifold pressure from cycle to cycle. The control strategyinvolves an evaluation of the output signals from multiple sensors,primarily an engine speed sensor, an engine load sensor and pressure andtemperature sensors in the intake manifold.

[0053] When the engine is switched to HCCI-mode the NO_(x) control loopS1 is initiated by a control unit. In a first step S2, the control unitreads the signals transmitted from a λ-sensor, an engine speed sensor,an engine load sensor, an intake manifold pressure sensor and an intakeair temperature sensor. Further sensors may include temperature sensorsfor engine oil and coolant.

[0054] When receiving the signals for engine speed and engine load, thecontrol unit will use these values to look up and select an associatedvalue for lambda λ_(s) in a map S3 stored in the control unit. Thecontrol unit will then check the signal from the intake manifoldtemperature T_(M) sensor and correct the selected λ-value S4, if themeasured temperature T_(M) deviates from a predetermined referencetemperature T_(REF) for said stored map. In general, if the intakemanifold temperature T_(M) increases the λ-value will need to becorrected upwards. Such a correction may also be required for increasingtemperatures for engine oil and coolant, which contributes to a generalincrease in temperature of the engine.

[0055] The control unit will then compare the selected λ_(s) with ameasured λ-value λ_(A) for the air fuel ratio in the exhaust gas S5. Ifλ_(A)>λ_(S) then the control unit will transmit a signal to the intakeair charging unit to decrease the absolute intake pressure S6.Similarly, if λ_(A)<λ_(S) then the control unit will transmit a signalto said air charging unit to increase the absolute intake pressure S7.

[0056] Before repeating the control loop, the control unit checks thatthe HCCI-mode is still in operation S8. If this is not the case, thenthe NO_(x) control ends S9.

[0057] In the above example the λ-value is measured using a λ-sensor,such as an oxygen sensor. However, it is also possible to use sensorsthat generate a signal indicative of the λ-value, such as aNO_(x)-sensor or an ion current sensor. Hence, for the step S5 above,either of a NO_(x) signal or ion current signal may be compared to arespective reference signal. If either signal is lower than itsreference signal, the control unit will transmit a signal to the intakeair charging unit to decrease the absolute intake pressure S6, and viceversa. According to a further embodiment, the value of λ may becalculated by the ECU, using the mass air flow (MAF) and the amount ofinjected fuel.

[0058] High λ-values combined with low fuel consumption and low NO_(x)emissions can not be achieved with atmospheric pressure in the intakemanifold during HCCI operation at high loads and/or speeds. In oneembodiment the engine is provided with an intake air charging unit,arranged to adjust the intake manifold pressure.

[0059]FIGS. 5A and 5B show different air charging arrangements for anengine E as described in connection with FIG. 1. According to FIG. 5A,the intake air charging unit may be a turbocharger 20 having acompressor 21 for intake air 22 and a turbine 23 driven by exhaust gases24 from the engine E, which turbine is provided with a wastegate forcontrolling the intake manifold pressure. According to one embodiment,this wastegate 25 may be connected to bypass the turbine 23. However,according to an alternative embodiment a wastegate 26 may be connectedto bypass the compressor 21, as indicated by dotted lines in the figure.A controllable valve 27 may also be connected to the intake conduitbetween the compressor 21 and the engine E, to exhaust compressed air tothe atmosphere.

[0060]FIG. 5B shows an alternatively intake air charging unit in theform of a supercharger 28 for intake air 29. As in the case of theturbocharger in FIG. 5A, the supercharger is provided with a throttle orwastegate for controlling the intake manifold pressure. A wastegate 30may be connected to bypass the supercharger 28, as indicated by dottedlines in the figure. A controllable valve 31 may also be connected tothe intake conduit between the supercharger 28 and the engine E, toexhaust compressed air to the atmosphere.

[0061] According to one embodiment, the intake air charging unit isarranged to increase the intake manifold pressure if the actual λ-valueis less than the selected λ-value. Similarly, the intake air chargingunit is arranged to decrease the intake manifold pressure if themeasured λ-value is greater than or equal to the selected λ-value.

[0062] The invention is not limited to the embodiments described aboveand may be varied freely within the scope of the appended claims.

1. An internal combustion engine provided with at least one cylinder and comprising: a fuel injector, through which fuel is injected into a combustion chamber, for each cylinder; a piston in the engine cylinder whose compression action causes a mixture of air and fuel within the combustion chamber to be ignited; at least one inlet valve for admitting gas which includes fresh air into said cylinder; at least one exhaust valve for exhausting combusted gases; a pressure sensor for measuring intake manifold pressure and generating a pressure signal; a sensor for generating a signal representing an actual λ-value; sensors for measuring at least one further engine operation parameter, and a control unit that controls fuel injection quantity per combustion cycle injected through each fuel injector, and during compression ignition mode, the control unit is arranged to select a λ-value from a map stored in the control unit, which value is a function of engine load and engine speed, and to compare the actual λ-value with the selected λ-value; whereby the control unit is arranged to adjust the intake manifold pressure as a function of the difference between the said λ-values in order to obtain the selected λ-value.
 2. The internal combustion engine according to claim 1, wherein the engine is provided with an intake air charging unit, arranged to adjust the intake manifold pressure.
 3. The internal combustion engine according to claim 2, wherein said λ-sensor is arranged to measure an air/fuel ratio and generate a signal representing the actual λ-value.
 4. The internal combustion engine according to claim 3, wherein the intake air charging unit is a turbocharger.
 5. The internal combustion engine according to claim 4, wherein the turbocharger is provided with a wastegate for controlling the intake manifold pressure.
 6. The internal combustion engine according to claim 3, wherein the intake air charging unit is a supercharger.
 7. The internal combustion engine according to claim 6, wherein the supercharger is provided with a throttle for controlling the intake manifold pressure.
 8. The internal combustion engine according to claim 3, wherein the intake air charging unit is arranged to increase the intake manifold pressure if the actual λ-value is less than the selected λ-value.
 9. The internal combustion engine according to claim 3, wherein the intake air charging unit is arranged to decrease the intake manifold pressure if the measured λ-value is greater than or equal to the selected λ-value.
 10. The internal combustion engine according to claim 2, wherein a NO_(x)-sensor is arranged to generate a signal representing the actual λ-value.
 11. The internal combustion engine according to claim 2, wherein an ion current sensor is arranged to generate a signal representing the actual λ-value.
 12. The internal combustion engine according to claim 1, wherein the engine is provided with a sensor for measuring current engine load and a sensor for measuring engine speed.
 13. The internal combustion engine according to claim 11, wherein the control unit is arranged to adjust the stored map of λ-values based a measured intake air temperature.
 14. The internal combustion engine according to claim 1, wherein the engine is provided with a sensor for measuring engine oil temperature and a sensor for measuring coolant temperature.
 15. A method for operating an internal combustion engine in a compression ignition mode of operation, the engine provided with at least one cylinder, the engine having a fuel injector, through which fuel is injected into a combustion chamber, for each cylinder; a control unit that controls fuel injection quantity per combustion cycle injected through each fuel injector; a piston in the engine cylinder whose compression action causes a mixture of air and fuel within the combustion chamber to be ignited during compression ignition mode; at least one inlet valve for admitting gas which includes fresh air into said cylinder; at least one exhaust valve for exhausting combusted gases; a sensor for measuring intake manifold pressure and generating a pressure signal; a sensor for generating a signal representing an actual λ-value; and sensors for measuring current engine load and engine speed, the method comprising: selecting a λ-value from a map stored in the control unit, which value is a function of engine load and engine speed; comparing the actual λ-value with the selected λ-value; and adjusting the intake manifold pressure as a function of the difference between the said λ-values in order to obtain the selected λ-value.
 16. The method according to claim 15 further comprising using a NO_(x)-sensor to generate a signal representing the actual λ-value.
 17. The method according to claim 15 further comprising increasing the intake manifold pressure if the measured λ-value is less than the selected λ-value.
 18. The method according to claim 17, further comprising decreasing the intake manifold pressure if the measured λ-value is greater than or equal to the selected λ-value.
 19. The method according to claim 18 further comprising measuring intake air temperature and adjusting the λ-values in said stored map with respect to the measured temperature.
 20. The method according to claim 15 further comprising adjusting the intake manifold pressure using an intake air charging unit.
 21. The method according to claim 20 further comprising using an intake air charging unit in the form of a turbocharger.
 22. The method according to claim 20 further comprising controlling the intake air charging unit using a throttle or a wastegate.
 23. The method according to claim 20 further comprising using an intake air charging unit in the form of a supercharger.
 24. The method according to claim 20 further comprising controlling the intake air charging unit using a throttle.
 25. A computer readable storage device having stored therein data representing instructions executable by a computer to implement a compression ignition for an internal combustion engine, the engine having a piston disposed in a cylinder to define a combustion chamber, intake valves for admitting fresh air into the cylinder, a fuel injector for injecting fuel into the combustion chamber, exhaust valves for discharging exhaust gas resulting from combustion within the cylinder, wherein opening and closing timings of the intake means and opening and closing timings of the exhaust means are adjustable, and sensors for measuring an actual λ-value for the exhaust gas and at least one further engine operating parameter, the computer readable storage device comprising: instructions for adjusting opening and closing timings of the intake means and opening and closing timings of the exhaust means such that the piston reciprocates within the cylinder to perform an exhaust phase, an exhaust gas retaining phase, an intake phase, a compression phase, and an expansion phase; instructions for providing a first start time of a first fuel injection by the fuel injector during said exhaust gas retaining phase and a second start time of a second fuel injection by the fuel injector during said exhaust gas retaining phase; instructions for selecting a λ-value from a map stored in the control unit, which value is a function of engine load and engine speed; instructions for determining a difference between the actual λ-value with the selected λ-value; and instructions for adjusting the intake manifold pressure as a function of the difference between the said λ-values in order to obtain the selected λ-value.
 26. The computer readable storage device according to claim 25 wherein the computer readable storage device further comprises instructions for determining intake manifold pressure control signals indicative of the adjustment required by an intake air charging unit, as determined by the control unit on the basis of comparison between the actual λ-value with the selected λ-value.
 27. The computer readable storage device according to claim 25 wherein the computer readable storage device further comprises instructions for determining intake manifold temperature by means of a temperature sensor and for adjusting the λ-values in said stored map with respect to the measured temperature.
 28. A method for controlling an engine having a variable valve actuator, the method comprising: operating the engine in a first mode where the engine carries out spark ignition combustion of air and fuel; changing operation to a second mode where the engine carries out homogenous charge compression ignition combustion of air and fuel, where said variable valve actuator unit is adjusted in response to said change; and during at least said second mode, controlling an air-fuel mixture in the engine by adjusting manifold pressure.
 29. The method of claim 28 wherein said variable valve actuator is a variable cam timing actuator.
 30. The method of claim 28 wherein said variable valve actuator is a variable valve lift actuator.
 31. The method of claim 28 wherein said variable valve actuator is a variable valve lift actuator is a cam profile switching actuator.
 32. The method of claim 28 wherein said adjustment to said variable valve actuator provides a shorter valve opening duration or lift.
 33. The method of claim 28 wherein said adjustment to said variable valve actuator provides increased negative valve overlap.
 34. The method of claim 28 wherein said adjustment of manifold pressure is via at least a compression device coupled to the engine. 